I hesitate to post because this is a hot issue with fly fishers and I am rather new here. But it is an issue with which I am well versed and which I have researched over a number of years. In all honesty, I believe it is an issue to which fishers react to emotionally and without any scientific data. I think it is very similar to fly fishers who believe that Catch and Release Fisheries need barbless hook regulations to preserve fisheries. More on that later.
When I started fly fishing, I used a stomach pump. I have found it to be an effective method of examining what the fish has taken without killing the fish. It is a good learning device and has been recommended by Dr. Carl Richards of "Selective Trout" fame.
Dr. Carl Richards wrote the chapter titled What Trout Eat
in the The Complete Guide to Fishing with a Fly Rod
, ISBN: 0-87165-013-4. I quote from the text, "If fish are feeding underwater, two methods can be used to discover what they are feeding on. The best way is to catch a fish (usually one dummy can be taken using an attractor, fished wet, such as a Coachman) and pump his stomach with a simple stomach pump." From the caption for the pictures, "Above, a stomach designed for trout is an effective way of discovering what the fish are feeding on without harming it
The most important thing to do is to not use it on smaller fish. You should not have to force the tube down. A stomach pump is a misnomer. It is actually a throat pump to remove what the fish has recently taken. Also turning the fish over will disorient the fish and keep it from struggling.
To those that say you have already caught the a fish and so you know what they are feeding on has simply not seen the fly used and what the fish are actually feeding on. I've never seen a Royal Wulff or Royal Coachman hatch but they do catch fish.
Like many techniques in fly fishing, I believe using a stomach pump is what could be termed a fairness issue.
Is it "fair" to sample what the trout has eaten to gain and edge in fly fishing? That is really the question that each of us has to answer.
Some fly fishers feel that nymphing is somehow unfair and some nymphers think nymphing with strike indicators is less fair than fishing without. If you don't think nymphing is "fair", don't. If you don't want to use a stomach pump, don't.
But stomach pumps are not
and never will be a resource issue. They have no impact on fish populations as I will demonstrate below.
Fish survival and population density studies for a health fishery have shown that there is an average annual mortality of 30% from natural causes. That is why C & R fishing does not impact a healthy fishery despite a mortality rate of about 3-4% for released fish. I doubt the usage of stomach pumps has any population impact, because they are used on occasion, whereas the 3-4% mortality applies to every fish caught.
The greatest unnecessary killers of C & R fisheries, I believe, are poor release technique and overplaying fish. Compared to these two, stomach pumps aren't even on the radar.
I personally believe the act of catching and releasing is more harmful than the act of pumping. Of course you need to catch a fish to pump it so the effects are additive. But were we to be given the choice of having our throats sampled or having a hook put in our jaw and dragged around to the point of exhaustion, most of us would choose the pump.
I believe that properly performed, throat sampling via a stomach pump is no more injurious than a properly performed C & R. When done incorrectly it can kill but so can C & R, and I suspect that there are hundreds of fish C & R'd for every fish pumped. I pumped two fish total several years ago and that was to show a beginner what the fish were feeding on.
Gentlemen, population effects are based on numbers and there are simply not enough fish pumped to have any effect compared to C & R mortality and natural mortality.
Even the use of barbed hooks does not affect trout populations in healthy fisheries. Dr. Robert Behnke,
one of the the leading trout researchers in the world, has said that many studies have shown that the mortality difference (of about 1.0%)
between barbless (3.5-4%) and barbed (4-5%) does not affect trout populations given the high natural mortality of trout.
See Dr. Robert Behnke's accolades:
Robert J Behnke Endowed Chair in Coldwater Conservation | FWCB Research & Outreach
Fly Rod and Reel Magazine 2003 January - 2003 Angler of the Year: Dr. Robert Behnke
Water Center - People - Robert J. Behnke
See the references below for barbless hooks vs barbed hooks.